Idle mode, discontinuous reception (DRX) mode and dormant mode are examples of energy-saving modes practiced in available mobile communication systems and expected to have equivalents in future systems. In an energy-saving mode, there are typically pre-agreed time periods (non-listening periods) during which no data will be transmitted from the network to the user equipments (UEs). During such time periods, in order to save energy, the UE may turn off its one or more radio receivers until the subsequent listening period begins or when an active (i.e., not explicitly energy-saving) mode is resumed. In some mobile communication systems, the characteristics of several energy-saving modes may be combined to reduce the energy consumption further, such as by applying DRX during dormant mode.
An energy-saving mode may be triggered by inactivity, such as when no data (e.g., no data packets) has been transmitted or received for a predefined amount of time. Typically all beam-related information is lost when the UE enters an energy-saving mode. When the UE leaves the non-listening period—when it either moves into a listening period or reenters active mode—it will therefore need to listen to a number of signals, in particular reference signals, to determine reliable beam-related information. According to proposals, such signals guiding adaptation may include Signature Sequence Index (SSI), Tracking RAN Area Signal (TRAS), Paging Indication Channel (PICH)/Paging Message Channel (PMCH).
In view of the large variety of requirements for the next generation of mobile communications system (“5G”), information relating to many different carrier frequencies will be needed. For example, low bands will be needed to ensure sufficient coverage and higher bands (e.g. at and above 30 GHz) will be needed to achieve the required capacity. At high frequencies the propagation properties are more challenging, and active beamforming may be required both at the base station and at the UE to reach a sufficient link budget. Higher frequencies generally increase the degree of directivity of UE antennas, which means that a single antenna element at high frequency typically will not offer omnidirectional or approximately omnidirectional coverage. It is therefore foreseen that multiple UE antenna elements with beam patterns pointing in different directions and with different polarizations will be preferred in order to improve the link budget and approach omnidirectional coverage.
UE beamforming is typically implemented by one of three basic approaches: analog, digital and hybrid (analog and digital beamforming combined), with respective advantages and challenges. Digital beamforming is the most flexible approach but also the costliest, mainly due to the large number of radios and baseband chains that it requires. Analog beamforming is the least flexible but typically cheaper to manufacture, as one radio and baseband chain may supply a plurality of antenna elements. Hybrid beamforming attempts to combine the advantages of analog and digital beamforming. Clearly the cost and performance requirements on a given UE will determine what beamforming approach will be applied.
As an effect of the expected multiplication of receive antennas per UE combined with the significantly broader frequency range foreseen for 5G, the acquisition of sufficient beam-related information at wakeup from a non-listening period will be a potentially demanding task for UEs. The embodiments disclosed herein seek to reduce the harmful impact that this task may have on overall UE performance.